1. Field of the Invention
This invention relates to a lens aperture-adjustment device for a closed circuit television camera and a closed circuit television camera.
2. Description of Related Art
Closed circuit television (CCTV) is transmission of a video signal so as to enable a person permitted to view to selectively receive the video signal in a predetermined closed range (namely, on the closed circuit) rather than transmission of a video signal to an indefinite number of persons like a TV broadcast. Systems using the CCTV include a crime prevention system (security system) using a camera installed in a convenience store, a street, a parking lot, etc., a monitor system of a factory line, and the like. Particularly, a security camera and a monitor camera used in the CCTV system are called CCTV cameras.
The CCTV camera is installed not only in facilities, but also outdoors as on the street, in a parking lot, etc, and picks up an image of an object at any time of day or night and thus needs adjustment of a lens aperture in response to the lightness of the imaging environment. To save a person watching an image picked up by the CCTV camera (who will be hereinafter referred to as “watchperson” from having to adjusting the lens aperture in response to change in the environment, most of the currently installed CCTV cameras have a function of automatically making a lens aperture adjustment based on the lightness of the imaging environment.
While lens aperture adjustment of the CCTV camera is made automatically, the watchperson might want to make a manual lens aperture adjustment. Thus, a lens aperture-adjustment device of a CCTV camera for enabling the user to switch between automatic adjustment and manual adjustment of lens adjustment is demanded. For example, JP-B-5-86707 and JP-B-6-42720 listed below disclose each a lens aperture-adjustment device of a CCTV camera for enabling the user to switch between automatic adjustment and manual adjustment of lens aperture adjustment:
A lens aperture of a CCTV camera is implemented as follows: A plurality of plates of metal, etc., called aperture blade are used in combination to form a roughly circular hole and the aperture blades are moved, thereby changing the diameter of the hole. Generally, as the number of the aperture blades increases, the hole becomes close to a circle and the accuracy of the lens aperture, namely, the accuracy of the quantity of light input to the CCTV camera in response to the imaging environment is enhanced.
However, a lens aperture-adjustment device of a CCTV camera according to the related art for enabling the user to switch between automatic adjustment and manual adjustment of lens aperture (which will be hereinafter referred to as “related art device”) uses a galvanometer as a motor for moving aperture blades and thus only two to three aperture blades can be moved at the same time. Thus, the related art device cannot provide the accuracy of the lens aperture at the required level of the current society.
To move the aperture blades, the related art device needs feedback control involved in the aperture position using an aperture position detection element and/or aperture speed using a tachogenerator, and an additional position feedback circuit and speed feedback circuit become required.
Further, to enable the user to switch between automatic adjustment and manual adjustment of lens aperture, the related art device needs to be provided with a plurality of switches and must control the switches.